Has the advantage of DRS been reduced in Formula 1?

DRS, the mechanism that allows drivers to attempt overtakes easier was very effective in 2011 and 2012. Nowadays it's common to see drivers being close behind a competitor lap after lap, unable to overtake. Why is this?

The biggest factor is that the power units and engine modules have become such that they can have more effect on straight line speed. Another factor are the changes in aerodynamics that have rendered the advantage of the DRS rear wing design to be less prominent than in the early 2010's.